JP2012107751A - Metal tube for thermal decomposition reaction - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metal tube for a thermal decomposition reaction which is excellent in both a heat exchange characteristic and a thermal decomposition reaction characteristic, and suitable for use in a precess for thermally decomposing hydrocarbons.SOLUTION: The metal tube for the thermal decomposition reaction in which three or four stripes of ribs 1 which are inclined at angles of 20 to 35° with respect to the pipe axial direction and spirally extended are formed at its internal peripheral face is characterized in that, when a rib height at the cross section of the rib 1 is set as h, a rib width at the bottom of a valley is set as w, and a valley-bottom inside diameter of the tube is set as Di, h/Di is 0.1 to 0.2, and h/w is 0.25 to 1.0.

Description

The present invention relates to a metal tube for thermal decomposition reaction having a rib on the inner peripheral surface of a tube suitable for a cracking furnace tube, a reforming furnace tube, a heating furnace tube or a heat exchanger tube in a petroleum refining or petrochemical plant. More specifically, for example, in an ethylene plant, heat suitable for use as a tube for producing olefin (C _n H _2n ) by causing a pyrolysis reaction to the hydrocarbons inside the tube from the heat applied from the outer surface of the tube. The present invention relates to a metal tube for decomposition reaction.

Olefins (C _n H _2n ) such as ethylene (C ₂ H ₄ ) are produced by pyrolyzing hydrocarbons (naphtha, natural gas, ethane, etc.). Specifically, hydrocarbons are placed inside a pipe made of a high Cr-high Ni alloy typified by 25Cr-25Ni or 25Cr-38Ni, or stainless steel typified by SUS304, etc., piped in the reactor. Is supplied together with water vapor, and heat is applied from the outer surface of the tube, whereby hydrocarbons are thermally decomposed on the inner surface of the tube to obtain olefinic hydrocarbons (ethylene, propylene, etc.).

  In the above thermal decomposition reaction, it is necessary to efficiently transfer the heat applied from the outer surface of the tube to the inner surface of the tube so that the hydrocarbons are not discharged out of the reaction furnace without being reacted. That is, the tube needs excellent “heat exchange characteristics”. This heat exchange characteristic can be evaluated by the average temperature of the fluid at the tube outlet. This average temperature is higher if the heat exchange characteristics of the tube are excellent.

  A mixed gas of hydrocarbons and water vapor supplied into the steel pipe is supplied at high speed from the pipe inlet at a low pressure. The unreacted mixed gas and the gas generated by the reaction move a long distance along the rib provided on the inner surface of the tube. Therefore, depending on the shape of the rib, the gas flow is inhibited by the rib, the fluid at the center of the tube and the fluid at the bottom of the rib valley are separated, and mass transfer (reaction) between the tube center and the bottom of the rib valley is insufficient. Become. When this happens, the reaction product stays in the valleys of the ribs and the pyrolysis reaction of the hydrocarbons proceeds too much, while the pyrolysis reaction of the fluid in the center of the tube is not sufficiently performed and the yield is reduced. Arise. In order to solve this problem, it is necessary that the tube has excellent “thermal decomposition reaction characteristics”. Since this thermal decomposition reaction characteristic depends on mass transfer in the pipe, it is evaluated by a temperature deviation at the pipe outlet.

Patent Document 1 (Japanese Patent Application Laid-Open No. 58-173022) discloses a method of manufacturing a tube with an inner surface spiral rib that has been manufactured by twisting after manufacturing a metal tube having straight ribs by hot extrusion. Further, Patent Document 2 (Japanese Patent Laid-Open No. 1-127896) discloses a concave curved surface having a curved radius of curvature R _F and a valley having a corrugated inner peripheral surface and forming a peak. Has disclosed a heat exchange tube material in which the curvature radius R _S satisfies the relationship R _S ≧ R _F.

  Further, Patent Document 3 (Japanese Patent Application Laid-Open No. 8-82494) discloses that a pipe shaft is attached to an inner surface of a pipe wall in one or a plurality of regions or whole areas in the tube axis direction from the entrance side end to the exit side end of the pipeline. A heat exchanging pipe is disclosed in which fins that cross each other are formed with an appropriate pitch. And patent document 4 (Japanese translations of PCT publication No. 2005-533917 gazette) is disclosing the pipe | tube which has a helical internal fin used for the process of thermally decomposing | disassembling a hydrocarbon in presence of a vapor | steam.

  However, the pipes having ribs and fins on the inner surface disclosed in each of the above patent documents are insufficient to improve both the “heat exchange characteristics” and the “thermal decomposition reaction characteristics”. Accordingly, there is a need for a heat exchange tube with internal ribs that further improves these characteristics.

  On the other hand, with regard to the use conditions of metal tubes for thermal decomposition reaction used for cracking furnaces for ethylene plants and the like, with the recent increase in demand for synthetic resins, the tendency of higher temperatures is becoming stronger from the viewpoint of improving ethylene yield. In such a pyrolysis reaction metal tube used at a high temperature, carbon is inevitably generated along with the pyrolysis reaction, and this carbon adheres to and accumulates on the inner surface of the tube. This is a phenomenon called “coking”.

  When coking occurs, the carbon deposited on the inner surface hinders the transfer of heat applied from the outer surface of the tube to the mixed gas, and the thermal decomposition reaction efficiency decreases. Also, the adhered and deposited carbon diffuses inside the steel pipe, causing so-called carburizing, embrittlement of the steel pipe, and damage from the carburized portion. Furthermore, if the adhered and deposited carbon peels off and accumulates in the steel pipe, not only the gas flow is blocked and the thermal decomposition reaction is hindered, but also the above damage is caused, and if the accumulation is significant, a serious accident such as explosion Cause. Therefore, in actual operation, so-called decoking work is carried out, in which air and water vapor are sent periodically to remove the deposited carbon by oxidation, but there are major problems such as stoppage of operation and increase of man-hours during the operation. become.

  The inner surface of the metal tube for thermal decomposition reaction is exposed to a carburizing atmosphere containing hydrocarbon gas or CO gas. Therefore, a heat-resistant material having carburization resistance and coking resistance in a carburizing gas atmosphere is required as a material for the pipe.

  In Patent Document 5 (Japanese Patent Laid-Open No. 2005-48284), the Cr layer has a Cr concentration of 10% or more and a thickness of 20 μm on the surface layer portion of a steel pipe made of a base material containing 20% to 35% by mass. Stainless steel pipes having carburization resistance and caulking resistance with a Cr-deficient layer of within are disclosed. Furthermore, although it is stated that protrusions and fins may be provided on the inner surface of the pipe according to the present invention, no specific shape is described.

JP-A-58-173022 Japanese Patent Laid-Open No. 1-127896 JP-A-8-82494 JP 2005-533917 A JP 2005-48284 A

The present invention has been made in view of the above circumstances, and an object thereof is to provide a metal tube for thermal decomposition reaction having the following characteristics (1) and (2).
(1) The unreacted gas in the central portion of the tube is frequently contacted with the inner surface of the tube, which is a reaction site, and has high thermal decomposition reaction characteristics.
(2) Excellent heat exchange characteristics as well as thermal decomposition reaction characteristics, and further excellent carburization resistance, and suitable characteristics for use in a process for pyrolyzing hydrocarbons.

  In order to solve the above-mentioned problems, the present inventors can increase the contact frequency of the unreacted gas in the central portion of the tube with respect to the inner surface of the tube, which is a reaction site, to promote the thermal decomposition reaction, Various studies were carried out to obtain a metal tube for thermal decomposition reaction having excellent heat exchange characteristics and carburization resistance, and the following knowledge (A) to (E) was obtained.

  (A) In order not to discharge hydrocarbons out of the reaction furnace in an unreacted state, it is necessary to efficiently transfer heat applied from the outer surface of the tube to the inner surface of the tube. That is, it is necessary that the heat exchange characteristics of the tube be excellent. For this purpose, it is necessary that the contact area between the gas flowing in the pipe and the inner surface of the pipe, that is, the inner surface area of the pipe is large.

(B) The inner surface area of the tube increases as the number of ribs formed on the inner surface of the tube is increased. Further, the higher the rib height, the larger the inner surface area, and the inner surface area of the rib having a sharply rising shape is larger than that of the gently wavy uneven shape in the tube cross-sectional direction.
The heat exchange characteristics when heated from the outer surface of the tube are improved when the rib has a sharp shape. If the rib has a sharp shape, the area of the thin portion of the tube, that is, the area of the bottom of the valley of the rib is large, and thus the heat exchange characteristics are increased. However, if the rib is too high, the distance from the apex of the rib to the outer surface of the tube increases. That is, the thickness of the tube when measured at the apex of the rib is increased, heat conduction from the outside of the tube becomes insufficient, the temperature of the peak portion of the rib is lowered, and the heat exchange characteristics are lowered.

  (C) A mixed gas of hydrocarbons and water vapor supplied to the inside of the pipe is supplied at high speed from the pipe inlet at a low pressure, and the gas generated by the reaction of the mixed gas is applied to the rib provided on the inner surface of the pipe. Move a long distance along. At this time, depending on the rib shape and the number of ribs, the gas flow is inhibited by the ribs, the velocity deviation between the fluid at the center of the tube and the fluid at the bottom of the rib valley increases, and the flow between the fluid at the center of the tube and the bottom of the rib valley When separated, mass transfer (reaction) between the tube center portion and the rib valley bottom portion becomes insufficient. As a result, the reaction product stays in the rib valleys and the thermal decomposition reaction of the hydrocarbons proceeds too much, and the reaction of the unreacted substances in the center of the tube is not sufficiently performed, resulting in a problem that the yield decreases. Therefore, it is necessary to reduce the residence of gas on the inner surface of the pipe and to make the gas flow uniform in the cross section. That is, it is necessary to improve the thermal decomposition reaction characteristics of the tube.

  (D) The higher the rib is, and the greater the inclination of the spiral rib from the tube axis direction, the better the thermal decomposition reaction characteristics of the tube. However, if the rib is too high or the slope of the helix is too large, the rib will obstruct the flow of the fluid at the bottom of the valley, and the fluid at the center of the tube and the fluid at the bottom of the rib will separate, increasing the fluid velocity deviation. , Thermal decomposition reaction characteristics deteriorate. Furthermore, as the number of ribs increases, the ribs obstruct the flow of fluid at the bottom of the valley and the flow of fluid to and from the center stagnate, and the fluid at the center of the tube and the fluid at the bottom of the rib separate and decompose. Characteristics are degraded.

  (E) For the above reasons, in order to achieve both heat exchange characteristics and thermal decomposition reaction characteristics, it is necessary to optimally select the number of ribs formed on the inner surface of the pipe, the height, the inclination angle from the pipe axis direction, etc. It is.

The present invention has been made on the basis of the above findings, and the gist of the following (1) to (4) is a metal tube for thermal decomposition reaction.

(1) By mass%, C: 0.01 to 0.6%, Si: 0.01 to 5%, Mn: 0.1 to 10%, P: 0.08% or less, S: 0.05% or less, Cr: 15 to 55%, Ni: 20 -70% and N: 0.001 to 0.25 %, the remainder has a chemical composition consisting of Fe and impurities, and extends in a spiral shape inclined at an angle of 20 to 35 ° with respect to the tube axial direction on the inner peripheral surface of the tube 3 It is a metal tube for thermal decomposition reaction in which ribs of 4 or 4 are formed, and in the cross section of the rib described above, the rib height is h, the rib width at the valley bottom is w, and the valley bottom inner diameter of the tube is Di. When h / Di is 0.1 to 0.2, h / w is 0.25 to 1.0, and the rib is formed integrally with the tube body by hot extrusion. Metal tube. The “rib cross section” is a section perpendicular to the tube axis.

(2) The metal tube for thermal decomposition reaction according to (1), further containing at least one component selected from the following (i) to (vi) by mass%.
(i) Cu: 0.01 to 5%, Co: 0.01 to 5%, 1 type or 2 types,
(ii) Mo: 0.01 to 3%, W: 0.01 to 6%, Ta: 0.01 to 6%, one or more,
(iii) Ti: 0.01 to 1%, Nb: 0.01 to 2%, 1 type or 2 types,
(iv) One or more of B: 0.001 to 0.1%, Zr: 0.001 to 0.1%, Hf: 0.001 to 0.5%,
(v) One or more of Mg: 0.0005 to 0.1%, Ca: 0.0005 to 0.1%, Al: 0.001 to 5%,
(vi) Rare earth element (REM): One or more of 0.0005 to 0.15%.

(3) The metal tube for thermal decomposition reaction according to (1) or (2 ) above, wherein the cross-sectional shape of the rib is an isosceles triangle.

(4) The metal tube for thermal decomposition reaction according to any one of (1) to (3), wherein the metal tube is a tube used for a process of thermally decomposing hydrocarbons.

  The cross section of the rib of the metal tube of the present invention can take various shapes such as a triangle shape and a trapezoidal shape. Of the triangular shapes, an isosceles triangular shape is desirable. In the trapezoidal shape, an isosceles trapezoidal shape is desirable. In the case of a trapezoidal shape, the longer of the two parallel sides is the valley bottom side.

  FIG. 1 is a partial cross-sectional view perpendicular to the tube axis for explaining the shape of the rib of the metal tube of the present invention. As shown, ribs 1 are provided on the inner surface of the tube. The shape of the rib illustrated here is an isosceles triangle. In the drawing, h is the height of the rib, and w is the rib width at the bottom of the valley. The rib valley bottom inner diameter Di is the inner diameter of the tube up to the rib valley bottom equivalent position, and the rib crest inner diameter Dm is the tube inner diameter up to the rib peak equivalent position. As will be described below, the isosceles triangle shape means a substantially isosceles triangle state.

  As described above, the cross-sectional shape of the rib provided inside the tube of the present invention can be various shapes such as a triangular shape and a trapezoidal shape. Here, the triangular shape and the trapezoidal shape include not only a triangular shape and a trapezoid shape in a strict sense, but also shapes that can be substantially regarded as a triangular shape and a trapezoid shape. For example, as shown in FIG. 1, the peak of the rib crest may be rounded. The same applies to the trapezoidal shape. The joint between the two parallel sides and the hypotenuse may be rounded, so-called chamfered. Further, the hypotenuse from the apex to the bottom of the valley of the rib is not necessarily a straight line. In particular, it is desirable to connect the hypotenuse and the bottom surface of the rib with a gentle curve.

  As described above, an isosceles triangle shape is desirable in the triangle shape, and an isosceles trapezoid shape is desirable in the trapezoid shape. Thus, if it is a symmetrical shape, it is easy to manufacture the pipe | tube with which the rib which is a continuous protrusion was provided in the inner surface by hot processing or cold processing.

  The metal tube of the present invention is a metal tube for thermal decomposition reaction having high heat exchange characteristics and thermal decomposition reaction characteristics. If this tube is used, the yield of olefins such as hydrocarbons can be increased with less energy. Moreover, since this pipe is excellent in coking resistance and carburization resistance, the operating rate of the manufacturing apparatus itself can be improved.

FIG. 1 is a partial cross-sectional view perpendicular to the tube axis for explaining the rib shape of the metal tube of the present invention. FIG. 2 is a diagram showing an average temperature and an average temperature difference of fluid at the tube outlet in metal tubes having different numbers of ribs and different inclination angles. FIG. 3 is a diagram showing the influence of the rib height and the inclination angle on the average temperature and the average temperature difference of the fluid at the tube outlet. FIG. 4 is a diagram showing the influence of the ratio (h / w) of the rib height h to the rib width w at the valley bottom and the rib inclination angle on the average temperature and the average temperature difference of the fluid at the pipe outlet. FIG. 5 is a copy of a photograph of a cross-section perpendicular to the tube axis of the manufactured tube.

1. About the shape of a rib In order to determine the said optimal rib shape, the following simulation test was done.

1-1. Simulation test 1
As shown in Table 1, metal tubes for pyrolysis reaction in which the number of ribs on the inner surface of the tube, the height, the shape, and the inclination angle were variously changed were produced, and simulations were performed under the conditions shown in Table 2.

  In the simulation, using the commercially available thermal fluid analysis program under the conditions shown in Table 2 without considering the thermal decomposition reaction, the mass conservation equation, the momentum conservation equation and the energy conservation equation for the fluid inside the steel pipe are used. The equations were combined and the flow and heat transfer behavior inside the steel pipe were evaluated by a three-dimensional heat flow analysis model, and the effective viscosity coefficient in the pipe, in other words, the effective thermal conductivity and effective diffusion coefficient were calculated. At this time, a turbulent flow model was used in order to consider the influence of turbulent flow. The result is shown in FIG.

  In FIG. 2, the horizontal axis is the average temperature of the fluid at the steel pipe outlet. The high average temperature means that the heat applied from the outer surface of the steel pipe is efficiently transferred, and means that the heat exchange characteristics are excellent.

  The vertical axis in FIG. 2 is the average temperature difference of the fluid at the steel pipe outlet. That this average temperature difference is small means that the temperature is uniformly distributed. In other words, the value of the average temperature difference being large means that the central portion of the steel pipe is cold and only the vicinity of the inner surface is in a locally heated state, and the thermal decomposition reaction characteristic is inferior.

The value on the vertical axis (average temperature difference) in FIG. 2 is expressed by the following equation when the average temperature at the tube outlet is T _mean (K) and the temperature at an arbitrary position on the same cross section is T _local (K). The required value ΔT. However, S is a cross-sectional area of the space through which the fluid in the pipe passes.

The following conclusion can be obtained from FIG.
1) The heat exchange characteristic represented by the average temperature of the fluid at the outlet of the pipe (horizontal axis in FIG. 2) is larger as the inner surface area of the pipe is larger. The inner surface area of the tube increases as the number of ribs increases.
2) The average temperature difference of the fluid at the pipe outlet (ΔT calculated by the above equation (1)) becomes smaller as the rib inclination angle is larger. That is, the thermal decomposition reaction characteristic increases as the rib inclination angle increases. If the inclination angle is the same, the thermal decomposition reaction characteristic becomes maximum when the number of ribs is three. That is, if the thermal decomposition reaction characteristics are arranged in descending order, the number of ribs is 3, the case of 4, the case of 2, the case of 5, the case of 1, the case of 1, and the case of Article 8.

1-2. Simulation test 2
As shown in Table 3, the number of ribs was three, the inclination angle and height of the ribs were changed, a simulation test was performed under the same conditions as in Table 2, and the influence of the rib shape was examined. The result is shown in FIG.

  As is apparent from FIG. 3, the average temperature shown on the horizontal axis increases as the height of the rib increases. That is, heat exchange characteristics are improved. In addition, the average temperature difference on the vertical axis is reduced, and the thermal decomposition reaction characteristics are also improved. However, when the rib height is 4.0 mm, the thermal decomposition reaction characteristic is poor. On the other hand, even if the rib height is increased to 10.0 mm, there is no significant difference in the thermal decomposition reaction characteristics compared to the rib height of 8.0 mm or 9.0 mm. Further, there is no significant difference in effect when the rib inclination angle is in the range of 25 ° to 35 °.

  As described above, the higher the rib height (h), the better the heat exchange characteristics and thermal decomposition reaction characteristics. However, if the rib is too high, the rib restrains the gas flow, the fluid at the bottom of the valley stays, and the thermal decomposition reaction characteristics deteriorate. Moreover, the temperature of a rib crest part falls and a heat exchange characteristic falls. In addition, coking tends to occur. Furthermore, it is difficult to form high ribs by hot extrusion or cold rolling. On the other hand, if the rib is too low, the inner surface area of the tube is reduced, the heat exchange characteristics are reduced, and the thermal decomposition reaction characteristics are also lowered.

1-3. Simulation test 3
As shown in Table 4, the number of ribs was 3, and the rib height was fixed at 5.5 mm, and the rib width w at the valley bottom was changed for each of the inclination angles 25 °, 30 °, and 35 °, A simulation test was performed under the same conditions as in Table 2. The result is shown in FIG.

  As is clear from FIG. 4, the smaller the h / w is, that is, the lower the shape of the mountain is, the more gently the wavy shape is, the lower the thermal decomposition reaction characteristics. That is, the average temperature difference on the vertical axis in FIGS. On the other hand, the larger the h / w, the better the thermal decomposition reaction characteristics. In addition, when h / w is small, the inner surface area is smaller than that of a sharp shape with a large h / w, so the average temperature on the horizontal axis in FIGS. That is, the heat exchange characteristics tend to decrease.

  In terms of tube production, if h / w is too large, in other words, if the rib is too thin and sharp, it is difficult to form a high rib by hot extrusion or cold rolling.

1-4. Determination of optimum rib shape based on simulation test
(1) Number of ribs Based on the results of simulation test 1, the number of ribs was 3 or 4. A more preferable number of ribs is three.
(2) Rib Inclination Angle From the results of Simulation Test 1, the rib inclination angle was 20 ° to 35 °. More preferred is 25 ° to 30 °.
(3) Rib shape (relationship between rib height h, rib width w at the valley bottom, and valley bottom inner diameter Di of the rib)
When the rib height at the cross section of the pipe is h, the rib width at the valley bottom is w, and the inner diameter of the valley bottom of the rib is Di, h / Di is 0.1 to 0.2, and the rib height h is at the valley bottom. The ratio (h / w) to the rib width w was set to 0.25 to 1.0.

  The rib height h is defined by h / Di. In other words, pipes of various sizes are used as the metal tubes for the pyrolysis reaction, but if the heat exchange characteristics and pyrolysis reaction characteristics of the fluid on the inner surface of the pipe are taken into account, the shape is similar. Good. Therefore, the rib height h may be defined by h / Di. In the result of the simulation test 2, as shown in FIG. 3, both the heat exchange characteristic and the thermal decomposition reaction characteristic are improved when the rib height h is 5.0 mm or more, and the rib height is improved as the rib height is higher. However, when the rib height is 8.0 to 10.0 mm, the higher the height, the better the heat exchange characteristics, but no significant difference is seen in the thermal decomposition reaction characteristics. On the other hand, it is preferable that the rib height is low because the rib can be easily formed and the tube can be easily manufactured. For the above reason, the preferable rib height h is 5.0 to 10.0 mm, and the valley bottom inner diameter Di of the pipe used in the simulation test 2 is 48 mm. 2. Also, as the rib height increases, it becomes more difficult to perform cold or hot rib molding, and therefore it is preferable to set the upper limit of the rib height to about 8 mm at which there is no difference in the thermal decomposition reaction characteristics in the simulation test. A more preferable upper limit of h / Di is 0.17.

Next, the relationship between the rib height h and the rib width w at the valley bottom will be described.
Considering the heat exchange characteristics and pyrolysis reaction characteristics of the pipe when heated from the outside (going back and forth between the valley fluid and the center fluid) and the workability of rib formation, the rib shape is defined only by the rib height h Instead, it is also necessary to define the ratio (h / w) between the rib height h and the rib width w at the bottom of the rib. As is clear from the results of the simulation test 3, the smaller the h / w, the larger the average temperature difference and the lower the thermal decomposition reaction characteristics. Therefore, the lower limit of h / w is set to 0.25. On the other hand, the larger the h / w, the better the thermal decomposition reaction characteristics. Therefore, a preferable lower limit of h / w is 0.35, and a more preferable lower limit is 0.4.

  On the other hand, in simulation test 3, the maximum value of h / w is only up to 0.46, but as h / w increases, both pyrolysis reaction characteristics (average temperature difference) and heat exchange characteristics (average temperature) improve. Tend to. Further, in simulation test 2 in which the rib height h was changed, h / w was not changed linearly, but as can be seen from the values of rib height h and valley bottom width w in Table 3, h / w Is changed from 0.28 to 0.84. As shown in FIG. 4, the larger the h / w, the better the thermal decomposition reaction characteristics (average temperature difference) and the heat exchange characteristics (average temperature). Was set to 1.0. A preferable upper limit is 0.7, and a more preferable upper limit is 0.55.

2. Manufacturing method of metal pipe of the present invention The metal pipe for pyrolysis reaction of the present invention is formed into a required pipe shape such as a seamless pipe or a welded pipe by means of melting, casting, hot working, cold working, welding or the like. Molded and manufactured. Moreover, you may shape | mold into a required pipe shape by methods, such as powder metallurgy and centrifugal casting.

The following (a) to (c) are exemplified as a method of forming a helical rib on the inner surface of the tube.
(a) a hot-extrusion pipe press provided with a mandrel in which a crest corresponding to a trough of the pipe and a trough corresponding to a rib of the pipe are formed in a direction parallel to the axial center line on the outer peripheral surface, or An internally straight ribbed tube having the same rib height in the longitudinal direction of the tube by a cold rolling mill provided with a mandrel having a crest and a trough similar to the above on the outer peripheral surface in a state parallel to the axis. Manufacturing. Next, the inner straight ribbed tube is twisted to form an inner spiral ribbed tube.
(b) A cold drawn pipe making machine having a plug formed in a spiral shape with a crest corresponding to the trough of the pipe on the outer peripheral surface and a trough corresponding to the rib of the pipe, the rib height is The same inner spiral ribbed tube in the direction.
(c) A rib is formed on the inner surface of the pipe by overlay welding in a spiral manner to obtain a pipe with an inner spiral rib.

  Among the above methods, in the manufacturing method in which the rib is integrally formed with the tube by hot extrusion, and then the spiral rib is formed by twisting, the tube is manufactured by powder metallurgy or centrifugal casting, Compared to the case where ribs are formed by prime welding, a long product can be manufactured, and even when a long tube of 10 m or longer is required, it is not necessary to weld the tubes to make a long product. Furthermore, the pipe manufactured by this method has the same material as the rib and mother pipe, and therefore has higher strength and corrosion resistance than pipes with ribs formed by overlay welding using different materials, such as hydrocarbons. It is suitable for applications that require high-temperature strength, corrosion resistance, and carburization resistance, such as pyrolysis reactors.

  In the method of forming ribs by hot extrusion, when the height of the rib is too high, the rib may be extruded without sufficiently following the mandrel shape, and a predetermined rib height may not be partially secured. . Therefore, in the production by the hot extrusion method, the shape of the rib that can be formed is limited, and it is not preferable that the rib is too high.

3. The material of the metal pipe of the present invention When the excellent carburization resistance and caulking resistance are strongly required, the following chemical composition is excellent in carburization resistance and caulking resistance, and also in high temperature strength and hot workability. It is preferable to have a tube having. In addition, "%" regarding component content means "mass%".

  (1) C: 0.01 to 0.6%, Si: 0.01 to 5%, Mn: 0.1 to 10%, P: 0.08% or less, S: 0.05% or less, Cr: 15 to 55%, Ni: 20 to 70%, N: A metal tube having a chemical composition of 0.001 to 0.25%, the balance being Fe and impurities.

(2) A metal tube containing, in addition to the above components, at least one component selected from at least one group of the following (i) to (vi).
(i) Cu: 0.01 to 5%, Co: 0.01 to 5%, 1 type or 2 types,
(ii) Mo: 0.01 to 3%, W: 0.01 to 6%, Ta: 0.01 to 6%, one or more,
(iii) Ti: 0.01 to 1%, Nb: 0.01 to 2%, 1 type or 2 types,
(iv) One or more of B: 0.001 to 0.1%, Zr: 0.001 to 0.1%, Hf: 0.001 to 0.5%,
(v) One or more of Mg: 0.0005 to 0.1%, Ca: 0.0005 to 0.1%, Al: 0.001 to 5%,
(vi) Rare earth element (REM): One or more of 0.0005 to 0.15%.

  The effect of each component described above and the reason for limiting the content will be described below.

C: 0.01 to 0.6%
C is effective to contain 0.01% or more in order to ensure high temperature strength. On the other hand, if it exceeds 0.6%, the toughness is extremely deteriorated, so the upper limit is made 0.6%. A more preferable range is 0.02% to 0.45%, and a further preferable range is 0.02% to 0.3%.

Si: 0.01-5%
Si is necessary as a deoxidizing element, but is also an element effective for improving oxidation resistance and carburization resistance. This effect is exhibited at a content of 0.01% or more. However, if it exceeds 5%, the weldability deteriorates and the structure becomes unstable, so the upper limit is made 5%. A more preferable range is 0.1 to 3%, and a most preferable range is 0.3 to 2%.

Mn: 0.1-10%
Mn is added for deoxidation and processability improvement. For this purpose, its content needs to be 0.1% or more. Further, since Mn is an austenite-forming element, it is possible to substitute a part of Ni with Mn. However, if it is excessively contained, workability deteriorates, so the upper limit is made 10%. A more preferable range is 0.1 to 5%, and a most preferable range is 0.1 to 2%.

P: 0.08% or less, S: 0.05% or less P and S are segregated at grain boundaries and deteriorate hot workability. Therefore, it is preferable to reduce as much as possible, but excessive reduction leads to an increase in manufacturing cost, so P is 0.08% or less and S is 0.05% or less. More preferably, P is 0.05% or less and S is 0.03% or less, and most preferably, P is 0.04% or less and S is 0.015% or less.

Cr: 15-55%
Cr is a main element for ensuring oxidation resistance and needs to be contained in an amount of 15% or more. Higher Cr content is preferable from the standpoint of oxidation resistance and carburization resistance, but excessive addition reduces the manufacturability of the tube and the structural stability at high temperatures during use, so the upper limit of content is 55%. And In order to prevent deterioration of the structure stability as well as workability, the upper limit is preferably made 35%. A more preferable range is 20 to 33%.

Ni: 20-70%
Ni is an element necessary for obtaining a stable austenite structure, and a content of 20 to 70% is required depending on the Cr content. However, excessive content causes high cost and difficulty in manufacturing the tube, so a more preferable range is 20 to 60%, and a most preferable range is 23 to 50%.

N: 0.001-0.25%
N is an element effective for improving the high-temperature strength. In order to obtain this effect, it is necessary to contain 0.001% or more. On the other hand, excessive addition significantly impairs processability, so the upper limit of the content is made 0.25%. A more preferable N content is 0.001% to 0.2%.

In addition, one or more of the following elements may be contained as desired.
One or two of Cu: 0.01-5%, Co: 0.01-5%
Cu and Co are effective in improving the high temperature strength in addition to stabilizing the austenite phase, and each may be contained in an amount of 0.01% or more. On the other hand, when each content exceeds 5%, hot workability is remarkably lowered. Therefore, the content is set to 0.01 to 5%. More preferable ranges are 0.01 to 3%, respectively.

One or more of Mo: 0.01 to 3%, W: 0.01 to 6%, Ta: 0.01 to 6%
Mo, W and Ta are all effective as a solid solution strengthening element for improving the high temperature strength. In order to exert the effect, the respective contents must be at least 0.01% or more. However, since excessive content inhibits deterioration of workability and structure stability, it is necessary to make Mo 3% and W and Ta each 6% or less. Any of Mo, W, and Ta is more preferably 0.01 to 2.5%, and still more preferably 0.01 to 2%.

One or two of Ti: 0.01-1%, Nb: 0.01-2%
Ti and Nb have a great effect on improving high-temperature strength, ductility and toughness even when added in a very small amount, but when the content is less than 0.01%, the effect cannot be obtained. If it exceeds 2%, workability and weldability deteriorate.

One or more of B: 0.001 to 0.1%, Zr: 0.00l to 0.1%, Hf: 0.001 to 0.5% B, Zr and Hf both strengthen grain boundaries, hot workability and high temperature strength properties Although it is an element effective for improving the content, any effect is not obtained when the content is less than 0.001%. On the other hand, if the content is excessive, the weldability is deteriorated, so that the content is set to 0.001 to 0.1%, 0.001 to 0.1%, and 0.001 to 0.5%, respectively.

One or more of Mg: 0.0005-0.1%, Ca: 0.0005-0.1%, Al: 0.001-5%
Mg, Ca and Al are all effective elements for improving hot workability, and the effect is obtained when Mg and Ca are contained in 0.0005% or more, and Al is contained in 0.001% or more. Al is also capable of significantly improving the carburization resistance of metal pipes when exposed to a carburizing gas environment because an oxide scale composed mainly of Cr and Al is produced. For this purpose, it is effective to contain 1.5% or more of Al. On the other hand, excessive addition of Mg and Ca deteriorates the weldability, so the upper limit of the content is 0.1% for Mg and Ca. On the other hand, if the Al content exceeds 5%, an intermetallic compound is precipitated in the alloy, so that toughness and creep ductility are significantly reduced.
A more preferable content range is 0.0008 to 0.05% for Mg and Ca, and 2 to 4% for Al when contained for improving carburization resistance.

Rare earth element (REM): One or more of 0.0005 to 0.15% Rare earth element is an element effective in improving oxidation resistance, but any of the contents less than 0.0005% cannot obtain the effect. Excessive addition reduces workability, so the upper limit of the content is 0.15%. The rare earth element means 17 elements obtained by combining Y and Sc with 15 elements of lanthanoid, and among them, it is particularly preferable to use one or more of Y, La, Ce and Nd.

4). Example of production of inner ribbed tube A straight ribbed tube having three or four ribs on the inner surface of the tube using a hollow billet having the composition shown in Table 5 and using a mandrel with irregularities corresponding to the rib shape. Manufactured by hot extrusion. The tube is subjected to a softening heat treatment at 1150 ° C., then twisted with an inclination angle of 27 ° from the tube axis direction, then heated to 1230 ° C. for 3 minutes, and then subjected to a product heat treatment that is water-cooled. A helically ribbed tube with dimensions of was obtained. A reproduction of a cross-sectional photograph of the tube is shown in FIG. As shown in the figure, no cracks in the ribs or cracks in the valleys were observed at all.

  Since the metal tube for pyrolysis reaction of the present invention has high heat exchange characteristics and pyrolysis reaction characteristics, not only can the yield of olefins such as hydrocarbons be increased with low energy, but also excellent coking resistance. The operating rate of the production apparatus itself can also be improved, and it can be used not only for the production of olefins such as ethylene, but also as a metal tube for thermal decomposition reaction used for all thermal decomposition reactions.

Claims (4)

In mass%, C: 0.01 to 0.6%, Si: 0.01 to 5%, Mn: 0.1 to 10%, P: 0.08% or less, S: 0.05% or less, Cr: 15 to 55%, Ni: 20 to 70% And N: 0.001 to 0.25 %, the remainder having a chemical composition composed of Fe and impurities, and extending in a spiral shape inclined at an angle of 20 to 35 ° with respect to the tube axis direction on the inner peripheral surface of the metal tube Or a metal tube for pyrolysis reaction in which four ribs are formed, and in the cross section of the rib, when the rib height is h, the rib width at the valley bottom is w, and the valley bottom inner diameter of the tube is Di ,
h / Di is 0.1 to 0.2, h / w is 0.25 to 1.0,
A metal tube for thermal decomposition reaction, wherein the rib is formed integrally with a tube body by hot extrusion.   2. The metal tube for thermal decomposition reaction according to claim 1, further comprising at least one component selected from the following (i) to (vi) by mass%.
  (i) Cu: 0.01 to 5%, Co: 0.01 to 5%, 1 type or 2 types,
  (ii) Mo: 0.01 to 3%, W: 0.01 to 6%, Ta: 0.01 to 6%, one or more,
  (iii) Ti: 0.01 to 1%, Nb: 0.01 to 2%, 1 type or 2 types,
  (iv) One or more of B: 0.001 to 0.1%, Zr: 0.001 to 0.1%, Hf: 0.001 to 0.5%,
  (v) One or more of Mg: 0.0005 to 0.1%, Ca: 0.0005 to 0.1%, Al: 0.001 to 5%,
  (vi) Rare earth element (REM): One or more of 0.0005 to 0.15%. The metal tube for thermal decomposition reaction according to claim 1 or 2, wherein a cross-sectional shape of the rib is an isosceles triangle. The metal tube for thermal decomposition reaction according to any one of claims 1 to 3, wherein the metal tube is a tube used for a process of thermally decomposing hydrocarbons.

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